To use the Finatra AsyncAppender first define all appenders that will log (i.e., Console or
RollingFile appenders) and wrap them in the custom AsyncAppender. For more guidance, see the Logback
documentation. There are also examples that use logback.xml files.

The counters track the number of discarded events by log level. The appender follows the standard
Logback AsyncAppender functionality for discarding events with the only addition being the
introduction of metrics.

Important

All the appender metrics are of Debug verbosity
and thus must explicitly be enabled. See the next section for information
on enabling the custom Logback AsyncAppender metrics.

Users need to create a JSON file and place it in the src/main/resources folder in
com/twitter/tunables/finagle/instances.json to whitelist the Logback metrics.
To whitelist all Logback metrics the JSON file could contain the following:

The configuration of each AsyncAppender will be added to the registry such that it can viewed
without accessing the statically defined configuration. Different AsyncAppenders will registered
in the registry by their defined name.